[Homer-users] Low pass filter error

[Grace Giles]

Yes, we are getting the error for 0.5 Hz on the LPF. We checked the time
variable and it is sampling every 500 sec (or a little over 500 sec) but so
close to 2 Hz that I would not expect the error.

We used this script
<http://www.physiologicalcomputing.net/?page_id=2621>convert Biopac's
.nir format to HomER's .nirs format, so the only other
issue I could think of would be one in the conversion. Are you aware of
anyone else having a similar problem? Would it be possible for someone to
quickly look at one of our files?

Many thanks,
Grace


On Wed, Mar 12, 2014 at 1:51 PM, David Boas <dboas at nmr.mgh.harvard.edu>wrote:

>  curious.
> you get the error for 0.5 Hz on the LPF? If so, then the time vector must
> not be presenting a sample at a 2 Hz sample rate.
>
> I could see the error happening at 1 Hz because of numerical round off
> errors.
>
> David
>
>
>
> On 3/11/14, 3:57 PM, Grace Giles wrote:
>
> Good afternoon,
>
>  I am using NIRS data obtained from the Biopac fNIRS system and when I
> attempt to enter filters in HomER (i.e. 0.00 high pass and 0.5, 1 or 2 low
> pass) I get the error message "Filter parameters exceed Nyquist frequency."
>
>  Biopac's sampling frequency is set at 2 Hz, which I believe according to
> Nyquist's Theorem means that I can only look at signals as fast as 1 Hz,
> but I am not sure why a filter of 1 is a violation.
>
>  Has anyone else using Biopac come across this issue? Any insight would
> be greatly appreciated. I am using HomER to analyze my data for the first
> time, so still fairly new.
>
>  Best,
> --
> Grace Giles
> PhD Candidate
> Nutrition and Behavior Lab
> Department of Psychology
> Tufts University
>
>
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-- 
Grace Giles
PhD Candidate
Nutrition and Behavior Lab
Department of Psychology
Tufts University
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